Engines may use a turbocharger to provide boosted intake air for improved engine torque/power output density. The turbocharger may include a compressor coupled to an exhaust-driven variable geometry turbine (VGT). The air flow through the compressor may be adjusted by changing geometry or position of the turbine vanes to improve the compressor operating range and the turbocharger efficiency. However, operating the turbocharger alone may not meet the high torque/power output demand.
One attempt to address the above issue includes an electric assisted turbocharger. One example approach is shown by Barthelet et al. in U.S. Pat. No. 7,779,634. Therein, an electrical motor is coupled to the turbocharger between the compressor and the turbine. The motor may provide additional boost to the engine to increase the operating range of the turbocharger. The motor may also operate as a generator to save energy.
However, the inventors herein have recognized potential issues with such systems. As one example, coupling the electric motor to the turbocharger may further complex the exhaust gas recirculation (EGR) rate and boost pressure control strategies. For example, both the EGR rate and the boost pressure may be affected by adjusting any one of the actuators including the VGT, the electric motor, and the EGR valve. An effective control strategy is required for operating the actuators to track the target EGR rate and the target boost pressure with low electric assistance, short response time, and low emission.
In one example, the issues described above may be addressed by a method comprising: a method comprising: responsive to a power deficiency below a threshold, adjusting a position of a turbine coupled to a compressor for a desired turbine efficiency, the compressor providing boosted air to an engine and the power deficiency being a difference between a target and an actual boost pressure; and responsive to the power deficiency above the threshold, adjusting the turbine position based on a target flow of exhaust gases recirculated back into the engine. In this way, the electric assisted VGT may be controlled to meet the target EGR rate and target boost pressure with low fuel consumption and reduced emission.
As one example, a power deficiency may be calculated based on the target boost pressure and the measured boost pressure. The engine operation may transition among three operation modes responsive to the power deficiency. For example, the engine operation may transition to a first mode responsive to the power deficiency being zero or negative. In the first mode, the VGT is adjusted to track the target boost pressure, and the EGR valve is adjusted to track the target EGR flow. The motor may not operate or may operate in the regenerative mode to conserve energy. The engine operation may transition to a second mode responsive to the power deficiency being positive and lower than the threshold. In the second mode, the VGT position may be adjusted to achieve the desired efficiency, which may be an optimal (maximum) turbine efficiency, the motor may be adjusted to track the target boost pressure based on the adjusted VGT position, and the EGR valve may be adjusted to track the target EGR flow. In this mode, the boost pressure is adjusted via the VGT and the motor, while the EGR flow is adjusted via the EGR valve. By adjusting the VGT to the desired turbine efficiency, energy consumption of the electric assisted turbocharger may be reduced. The engine operation may transition to a third mode responsive to the power deficiency being the same or higher than the threshold. In the third mode, both the VGT position and the EGR valve position may be simultaneously adjusted to track the target EGR flow. The motor may then operate to track the boost pressure based on the adjusted VGT position. As such, the boost pressure is adjusted by the motor, while the EGR flow is adjusted by the VGT and the EGR valve. By adjusting the VGT position to track the target EGR flow, high EGR flow demand during high power deficiency (such as during a transient tip-in) may be met. By grouping the actuators differently responsive to the level of power deficiency, the engine may quickly reach the target working point. Further, the fuel economy may be optimized by limiting the electric assistance and increasing the turbine efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.